This proposal, entitled ICD Therapy in Patients with Congestive Heart Failure, examines the current use of primary prevention implantable cardioverter defibrillator (ICD) in clinical practice. Although randomized clinical trials have established the ICD as the best therapy currently available to prevent sudden cardiac death (SCD) in patients with congestive heart failure (CHF) and systolic dysfunction, many uncertainties exist about the use of ICD therapy for the primary prevention of SCD in patients seen in routine clinical practice. These uncertainties involve the degree to which the results of clinical trials apply to clinical practice, the extent to which the current use of ICD therapy complies with evidence-based guidelines, and the characterization of patients who derive most benefit from ICD therapy. We have three specific aims: Aim 1: To determine whether the estimated benefit for ICDs implanted for SCD primary prevention in a large national registry is of comparable magnitude to that observed in primary prevention clinical trials of ICD therapy. We will compare mortality and complications following ICD implantation in patients enrolled in primary prevention clinical trials with those of patients enrolled in the national ICD Registry. Long-term outcomes for all analyses (including those for Aims 2 and 3) will be determined using the Medicare Claims Database for Medicare patients and the National Death Index for non-Medicare patients. Aim 2: To assess the extent of under-utilization and over-utilization of primary prevention ICD therapy in the United States. We will determine the number of ICD implants in CHF patients who are indicated for an ICD based on the current evidence-based guidelines using the American Heart Association's (AHA) Get With The Guidelines for CHF (GWTG-HF) database. We will investigate temporal changes in ICD use and variations in ICD use by patient characteristics. We will also determine the number and characteristics of patients who receive an ICD outside the current guidelines using the ICD Registry. Aim 3: To examine the effectiveness and safety of ICD therapy in key patient subgroups excluded from or not well-studied by the major primary prevention clinical trials. Using the ICD Registry and the AHA GWTG-HF database, we will examine the effectiveness of ICD therapy in the following subgroups: patients with left ventricular ejection fraction between 30% and 35%, patients who are within 40 days from a myocardial infarction and patients who are within 3 months from coronary revascularization. When completed, this project will provide novel evidence about the use of primary prevention ICDs that will support improved efficiency and effectiveness of this therapy at a national level.